1. Field of the Invention
The present invention generally relates to solar cell fabrication. More particularly, the present invention relates to systems and methods for preparing metallic precursor thin films for the growth of semiconductor compounds to be used for radiation detector and solar cell fabrication.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. One way of reducing the cost of solar cell electricity generation is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Compounds of Copper (Cu), Indium (In), Gallium (Ga), Selenium (Se) and Sulfur (S) which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1−xGax(SySe1−y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that have yielded conversion efficiencies approaching 20%. The structure of a conventional CIGS(S) photovoltaic cell is shown in FIG. 1. A device 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. An absorber film 12, is a CIGS(S) layer and is grown over a conductive layer 13, which was previously deposited on substrate 11 and which acts as the electrical contact for device 10.
Various conductive layers comprising Molybdenum (Mo), Tantalum (Ta), Tungsten (W), Titanium (Ti), their nitrides and stainless steel have been used in the solar cell structure of FIG. 1. If substrate 11 itself is a properly selected conductive material, it is possible not to use conductive layer 13, since substrate 11 may then be used as the ohmic contact to device 10. After absorber film 12 is formed, a transparent layer 14 such as a Cadmium Sulfide (CdS), Zinc Oxide (ZnO) or CdS/ZnO stack is formed on absorber film 12
Radiation 15 enters device 10 through transparent layer 14. Metallic grids (not shown) may also be deposited over transparent layer 14 to reduce the effective series resistance of device 10. The typical electrical type of absorber film 12 is p-type, and the typical electrical type of transparent layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The typical device structure of FIG. 1 is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the CIGS(S) absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of device 10 shown in FIG. 1.
In a thin film solar cell employing CIGS(S) absorber, the cell efficiency is a strong function of the molar ratio of Cu/(In+Ga). The Ga/(In+Ga) molar ratio also affects the performance of the solar cell. For good device performance Cu/(In+Ga) molar ratio is kept at or below 1.0. As the Ga/(Ga+In) molar ratio is increased, on the other hand, the optical bandgap of the absorber layer increases increasing the open circuit voltage of the solar cell. Consequently, it is desirous, but not required, for a thin film deposition process to have the capability of controlling the above mentioned molar ratios.
One prior art method described in U.S. Pat. No. 4,581,108 utilized an electro-deposition approach for metallic precursor preparation and reaction of the metallic precursor with Se to form the compound. In this method a Cu layer was first electrodeposited on a substrate. This was then followed by electro-deposition of an In layer and heating of the deposited Cu/In stack in a reactive atmosphere containing Se. It was claimed that, through this approach thickness of individual constituent layers is independently controlled providing good compositional control for the overall film. In practice, however, this technique was found to yield CuInSe2 films with poor adhesion to the Mo contact layer. In a publication (“Low Cost Methods for the Production of Semiconductor Films for CuInSe2/CdS Solar Cells”, Solar Cells, vol:21, p.65, 1987) electro-deposition and selenization of Cu/In and Cu/In/Ga layers were demonstrated for CIS and CIGS growth. One problem area was identified as peeling of the compound films. The cross-section of Mo/CuInSe2 interface obtained by SEM clearly showed a weak interface.
Another conventional technique used for CIGS(S) formation involves sputter deposition of Cu—Ga alloy followed by sputter deposition of an In layer to obtain a (Cu—Ga alloy/In) precursor stack on the Mo back contact (see U.S. Pat. No. 6,092,669). The stack is then reacted with selenium and/or sulfur to form the compound. This approach has the drawback of high cost. Material utilization in a sputtering technique is much lower than 100% and Cu—Ga target preparation is costly.
Therefore, there is still a need to develop a cost effective approach to form high-quality, well-adhering Cu(In,Ga)(Se,S)2 compound thin films with macro-scale as well as micro-scale compositional uniformities and Ga/(In+Ga) molar ratios in the range of 0.2-0.4.